The present invention relates to apparatus for the precision placement of electronic components on a hybrid circuit substrate and, more particularly, to the placement of small articles such as semiconductor chips, capacitor chips and integrated circuit chips, having solder bumps on their lower surfaces and generally known as flip-chips, on a ceramic substrate which has been preprinted with a thick film conductor pattern.
As the name suggests, hybrid circuits are a combination of discrete and integrated circuit techniques. As in integrated circuits, conductors, resistors and conductive lands are printed on a ceramic substrate. In thick film technology, the printed elements are generally several mils thick. Then discrete chips are precisely positioned over the conductive lands and subsequently bonded in position in a manner to complete the electrical circuit. The printed conductor lands provide a pattern which precisely matches to the solder bumps on the bottom surface of the flip chip and these bumps connect to the circuit elements within the chip. The bonded chips and substrate, with an exposed lead frame, are frequently encapsulated in toto in a potting compound for protection against physical and environmental damage. Use of unencapsulated chips on the circuit board allows for the manufacture of physically smaller circuits than those where discrete components which have already been encapsulated have their leads inserted into circuit boards fitted with receiving connectors or into predrilled holes wherein the leads are subsequently cut and clinched. A primary advantage of chips is their small size, some being nearly microscopic. Chips in the order of 0.030 by 0.030 inches square and 0.010 thick and solder bumps and conductor lands in the order of 0.005 inches in height and width, and spaced apart by similar distances, are not uncommon. Nevertheless, for the hybrid circuit technique to be successful, the small chips must be positioned and oriented such that when placed on the substrate, all bumps and lands are properly connected without error. This requires a high degree of precision in positioning which was achieved in early development of these techniques by human operators using microscopes and tweezers.
The need for automatic, rapid, precise, repeatable and low cost means to position and bond chips on substrates was apparent if the burgeoning requirements of mass production in the electronics industry were to be met. Generally speaking, in the apparatuses which have been developed in the past, the chip or other small component, e.g., beam leaded components, are picked up and placed by a hollow probe device which is connected to a vacuum source. When the probe touches the upper flat surface of the chip, the vacuum within the probe holds the chip against the probe end. The chip is then raised, translated to the substrate, and lowered onto the substrate. Permanent bonding of chip to substrate is accomplished in some systems while the probe continues to hold the chip. In other systems, the conductive lands are pretreated with some form of tacky adhesive, e.g., soldering flux. The probe gently presses the solder bumps on the lower chip surface into the tacky adhesive so that electrical contact is made with the conductive lands. Then the vacuum within the probe is released and the chip remains adhered to the substrate as the probe is withdrawn. A positive gas pressure within the probe is sometimes used to separate the chip from the probe. U.S. Pat. Nos. 3,453,714; 3,337,941; 3,657,790 are among the many which disclose a vacuum probe. U.S. Pat. No. 3,887,998 discloses a magnetic probe for holding chips.
The means to achieve precision placement of chips on substrates varies considerably through the prior art; but in virtually every case, the design approach results in a system which is extremely limited in flexibility. In those designs where a degree of flexibility is achieved, it is achieved at the expense of complexity, or mere multiplication, of work stations in the assembly flow path.
Broadly speaking, the prior art designs fall generally into two categories. In the first category, the substrate and the chip are both separately, fixedly and precisely oriented and located. A transfer mechanism, usually utilizing a vacuum probe as described above, travels an invariable, repetitive path to pick up the chip and place it at one selected position on the substrate. Then, a new substrate and new chip are fed into their respective positions and the operation repeats.
In the second category, the chips start out with a degree of disorientation, for example, at random in a vibratory feeder bowl. The feeder bowl, in the known manner, operates to bring each chip in turn to a precise position. From that point, the design is similar to the first category; although additional steps to angularly orient the chip may be required intermediate the feeder bowl and the precisely located substrate.
U.S. Pat. No. 3,337,941 discloses a complex apparatus which performs a plurality of steps to precisely orient a randomly oriented chip component before placing it upon a precisely located substrate. However, all such devices (in both categories) during a given operating run, can only place one chip at one specific location on each substrate as the substrates move along a synchronized conveyor. And each substrate is identical. Thus, it appears that another complete and similar arraratus would be required to place a second and different chip upon that same substrate and so forth. Or the apparatus could be set up again to handle the same substrates for a second pass after new chips were supplied and after the substrates were repositioned on the conveyor so that a later chip is not placed upon the earlier chip. All of this makes for an inflexible system albeit necessitated by the need to properly orient components before placements.
U.S. Pat. No. 3,909,933, a combination of the first and second categories, discloses an apparatus having a higher degree of flexibility in that it places a plurality of diverse component chips onto a substrate, each in a precise position. A tray of precisely positioned chips rests upon a table movable in the X-Y plane. The substrate rests upon another table movable in the X-Y plane. A pick up head translates in an invariable path along an X axis between the chip tray and the substrate. At the tray the head descends to pick up whichever selected component lies directly below; after translation to the substrate the pick-up head descends to deposit the selected chip on the substrate for bonding. In accordance with a numeric control system, the tray is automatically moved to the X-Y coordinates which place the selected chip component precisely at the location where the head will descend for a component pickup. The same control system locates the substrate by X-Y coordinates such that the area for receiving the selected component is precisely beneath the position where the translated head invariably descends. Thus by moving both the tray and the substrate, a prearranged assortment of components in a tray can be placed one at a time onto a single substrate. In U.S. Pat. No. 3,909,933, two individual trays and two pickup heads operate alternately under one control program to place components on the same substrate. In this apparatus, the trays and the substrate must be precisely located and the chips within a tray must be precisely located one in relation to the other, otherwise the head moving along its invariable path will fail to precisely place component on substrate. Telescopes, television cameras, micrometer adjustments, and the like are utilized in an effort to assure accuracy.
The prior art recognizes a further problem, that of orienting and centering a chip component already being carried on the pick up probe just prior to placement on the substrate. Some systems, as described above, rely on precise positioning of a component before pickup and precise location of the substrate, and assume that action of the pickup head will not disturb the known location and orientation of the component during pickup and translation between pickup station and placement station. In U.S. Pat. No. 3,337,941, a network of feeler switches senses the location of the solder balls on the underside of a chip; the chip is then angularly oriented on the vacuum probe to the desired position. This approach is limited in that one switch network can only "recognize" one pattern of solder balls on a chip and only if the chip is already in one of several probable orientations.
Another device for centering a chip on the vacuum probe prior to placement is disclosed in U.S. Pat. No. 3,982,979. Therein, the rectangular component is supported from below on a probe using a slight vacuum. The probe is centered in a four-sided cavity having the form of an inverted truncated pyramid. As the probe is lowered, the component makes contact with the cavity walls and becomes aligned thereto; at the same time, the component is centered on the probe. A substrate is precisely positioned above the cavity, and the probe is raised to position the centered component on the substrate from below.
What is needed is an apparatus for placement of chips, e.g., integrated circuit chips, capacitor chips, on a preprinted circuit board substrate of the thick film construction. In accordance with an automated program, the apparatus should be capable at a single work station to place with a high degree of precision a plurality of different chips of various types and physical and electrical sizes on a substrate as is required to complete the circuit. The apparatus should be capable of performing without mechanical modification to complete a variety of circuit boards in accordance with a plurality of readily modified programs. The apparatus should contain supplies of chips of many diverse types and sizes, always available for selection and placement on the substrate. Precise location of stored chips should not be required; the apparatus should orient and center each chip after selection and prior to placement.